1. Field of the Invention
The present invention relates to an active type range finder that measures a distance to an object by projecting a signal and receiving a reflected signal back.
2. Description of the Related Art
Active type range finders used in cameras and the like, which have employed the principle of triangulation, have been widely known in the art. In this method, the distance to an object is measured by projecting a light signal to the object and receiving the reflected signal.
In addition, in recent years, range finders of this type capable of measuring a distance without moving parts have also found increasing use. Such range finders use a semiconductor position sensing device (PSD), employed as a light receiving means, for receiving the above-mentioned reflected light.
Since this PSD outputs a signal corresponding to the "center-of-gravity" position of the received light spot, image uniformity of the projected light image is not required and a wide measuring range can be set. One of the drawbacks, however, is that resistance noise occurs because a PSD has a resistance layer. The S/N ratio deteriorates in the case of measuring the distance to a relatively far object because the quantity of reflected light decreases, lowering the distance measurement accuracy. A decrease in the distance measurement accuracy for distant objects has become quite a serious problem in recent years, particularly with the advent of cameras using zoom lenses which can take a photograph of a far object.
A method has been considered in which each one of a pair of silicon photocells (SPC) is used separately and the position at which the reflected light is received is detected by the ratio of their outputs. However, if an attempt is made to cover the whole range of measuring distances from near to far by this SPC pair, it is impossible to lengthen the base length of the range finder. This method is problematical because the overall distance measurement accuracy is decreased.